The present invention relates to a device for testing vehicle shock absorbers with at least one receptacle for receiving a wheel of a vehicle, means for exciting oscillations in the received wheel, and means for determining the amplitude of the oscillations at various frequencies as well as an evaluation and output unit.
Devices for testing vehicle shock absorbers are known, e.g. from the booklet xe2x80x9cBOGE SHOCKTESTER.xe2x80x9d The booklet describes a stationary test station onto which a vehicle can be driven with two wheels simultaneously. A wheel receptacle adapted to oscillate vertically up and down initiates movement of the axle mass. After the drive is disconnected, the oscillation of the axle diminishes gradually and passes through the a resonance of the axle. The amplitude of the oscillation at resonance is physically and mathematically related to the dampening. The oscillation amplitude is received by an excursion sensor and displayed or recorded, respectively. The result of the measurement disadvantageously depends to a great extent on the oscillating system of the apparatus.
German Pat. No. DE-A-43 05 048 discloses a method for testing a shock absorber of an automobile, whereby a value referred to as a xe2x80x9cdampening coefficientxe2x80x9d is determined using a parameter estimation method. The estimated dampening coefficient is compared with a reference value to determine if the deviation between the estimated dampening coefficient and the reference value is within a tolerance range.
This method is therefore limited to determining of a single value of the system dampening and to comparing this single value with a reference value. The evaluation is furthermore based on an estimation method. It is therefore not possible to use this method for effectively assessing the condition of a shock absorber over a wider operating range.
U.S. Pat. No. 3,902,353 describes a method for testing shock absorbers installed in an automobile wherein a quantity related to the xe2x80x9cdampening efficiency of a shock absorberxe2x80x9d is determined, namely the phase between the excursion and the force of the wheel receptacle. This method, however, has the disadvantage that components, such as various masses and springs, are assumed to be known, which in reality may not the case, for example in an automobile repair facility not dedicated to a specific vehicle brand. Moreover, only mass-produced vehicles with specified tires and air pressure can be tested.
U.S. Pat. No. 4,103,532 describes another method for testing shock absorbers, wherein an xe2x80x9cequivalent viscous dampening coefficientxe2x80x9d of one of the wheel suspensions of an automobile is determined. This equivalent dampening coefficient describes the dampening characteristics of the wheel suspension of a wheel, including the additional friction forces between the individual elements. Disadvantageously, however, although the measurement result includes effects from the other elements of the wheel suspension, these effects cannot be separated out. For example, if the dampening of a wheel suspension determined with this method is too small, then it cannot be determined why the dampening is too small. This result may be caused by a tire spring rate which is too high, or by a wheel mass which is too high.
The measurements obtained with the aforedescribed methods disadvantageously provide results only in the oscillation range of the wheel, without including the characteristics of the initial oscillation. It should be noted, however, that the mass of the body (load) of a vehicle or the spring rate of the body (lowering of the body) are equally important, since the shock absorber should also dampen the oscillations in this oscillation range.
It is thus an object of the present invention to provide an improved device for testing shock absorbers, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved device for testing shock absorbers essentially independent of the tire pressure and the masses.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by providing a mechanism for determining the characteristics of the shock absorber while a wheel is mounted on a vehicle, wherein the mechanism includes means for determining the force supplied via the wheel receptacle to the oscillating system, which includes the shock absorber and is the spring of the vehicle.
Heretofore, the characteristics of a shock absorber, i.e. a force-velocity diagram, could only be tested on shock absorbers in special test stations after the shock absorbers are removed from the vehicle. When the shock absorbers are installed, only single values can be measured and compared with reference values, which is not sufficient to assess the condition of a shock absorber. According to one advantageous feature of the invention, that measurement values can be obtained easily and inexpensively, while side effects that cannot be controlled (such as the oscillating engine mass) do not significantly affect the measurement results.
According to another feature of the invention, a mechanism is provided for determining the power supplied via the wheel receptacle to the oscillating system which includes the shock absorber and the spring of the vehicle.
According to another feature of the invention, sensors are provided on the wheel hub, the wheel receptacle, and the body of the vehicle.
According to yet another feature of the invention, a force sensor is provided on the wheel receptacle, and an excursion and/or angle sensor is provided on the mechanism for initiating oscillations of the wheel. These features are provided to determine the characteristic values and characteristic curves, respectively, of m1 (body mass), m2 (wheel mass), c1 (body spring), c2 (wheel spring), and k1 (shock absorber). The other sensors disposed on the masses are not required, which advantageously makes the device efficient to operate.
Advantageously, means for recording and evaluating measurement data to produce separate values of the pull and push stage are provided.
According to yet another feature of the present invention, the excitation stroke of the mechanism for initiating oscillations in the wheel is variable, which allows an accurate determination of the amplitude at the resonance frequency.
According to another feature of the present invention, the sensors on the wheel hub, on the receptacle and the body of the motor vehicle, enable measuring of the spring constant of the elements of the wheel suspension. In this manner, values for determining the resonance frequency of the body and the relative movement between the body and the wheel hub, respectively, can be provided.
According to another feature of the present invention, means are provided for adjusting the level of the vehicle body.
Advantageously, the device is movable.
According to another feature of the present invention, an adjustable eccentric is provided for varying the excitation stroke.
Suitably, the output unit includes a display and/or printer.